In this work, the phenylsulfonic acid functionalized SBA-15 silica was prepared by the reaction of mesoporous SBA-15 silica with phenyltrimethoxysilane, followed by the sulfonation with chlorosulfonic acid. The functionalized SBA-15 silica was then tested as a heterogeneous acid catalyst in the esterification of free fatty acid (FFA) present in vegetable oils with methanol. The textural feature and Structure of the solid catalyst were investigated by using Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), small-angle X-ray scattering (SAXS), thermo gravimetric analysis (TG) and nitrogen adsorption-desorption techniques. When 7 wt.% of the solid catalyst and 15:1 M ratio of methanol to oil were used in the esterification reaction, the FFA conversion to methyl esters of 96.7% was attained after 5 h of reaction at a temperature of 67 degrees C. It is shown that the heterogeneous acid catalyst possesses a good stability. (C) 2013 Elsevier B.V. All rights reserved.